A Derivation of the Co-Channel Reuse Distance - A Devrivation of

A Derivation of the Co-Channel Reuse Distance
for Stations in the
Medium Frequency Narrowband Area Service
(MF NAS)
Spectrum Planning and Engineering Team Document: SP 5/04
Radiofrequency Planning Group Date: April 2004
Australian Communications Authority
A Derivation of the Co-Channel Reuse Distance for Stations in the
Medium Frequency Narrowband Area Service (MF NAS)
TABLE OF CONTENTS
1. INTRODUCTION ..................................................................................................................................1
2. BACKGROUND .....................................................................................................................................1
3. MF BROADCASTING SERVICE PLANNING..................................................................................2
4. DETERMINING AN MF NAS CO-CHANNEL REUSE DISTANCE ..............................................2
4.1 Minimum Wanted Signal Level.............................................................................3
4.2 Calculation of the Protected Unwanted Signal Level ............................................3
4.3 Sources of the Unwanted Signal ............................................................................3
4.4 Allowance for Multiple Interference Sources........................................................4
4.5 The Level of Protection from Co-Channel Sources...............................................4
4.6 Calculation of the Minimum Co-Channel Separation Distance ............................5
5. DISCUSSION..........................................................................................................................................6
6. CONCLUSION .......................................................................................................................................7
BIBLIGRAPHY ...............................................................................................................................................7
Appendix 1 – MF NAS Wanted Signal Level and Coverage ............................................8
Appendix 2 – Consideration of Interference from Multiple Co-Channel Transmitters...11
Appendix 3 – Extract from the ABA Planning Parameters .............................................12
Appendix 4 – Extract from ITU-R P.382-2 .....................................................................13
Appendix 5 – Extract from ITU-R P.368-7 .....................................................................14
Appendix 6 – MF Propagation.........................................................................................15
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Spectrum Planning Report SP 5/04 i April 04
A Derivation of the Co-Channel Reuse Distance for Stations in the
Medium Frequency Narrowband Area Service (MF NAS)
1. INTRODUCTION
This report explores the derivation of the minimum separation distance between co-channel
MF NAS transmitters. This minimum co-channel separation distance is part of the
simplified coordination check carried out as part of the MF NAS licence application
procedure. The licensing of the operation of MF NAS transmitters has been designed to be
an “over-the-counter” licensing procedure. The procedure makes use of simple coordination
checks rather than a detailed planning and engineering assessment for each transmitter.
The use of simple coordination checks, because of their general application, must be
designed to protect existing services under worst case conditions. This means that in most
cases a distance less than the specified minimum co-channel separation distance could
provide the required level of protection. This paper also examines the range over which the
actual co-channel separation distance can vary. This paper does not however address the
question of whether in processing MF NAS licence applications the Australian
Communications Authority (ACA) should take into account the higher level of technical
analysis and planning outlined in the paper. Such consideration would need to take into
account ACA policy and resource issues that are beyond the scope of this planning report.
2. BACKGROUND
MF NAS stations typically deliver narrowcasting programme content1 using frequencies in
the band 1606.5 to 1705 kHz. This band is located just above the Australian Medium
Frequency (MF) broadcasting services band 526.5-1606.5 kHz that is used for Amplitude
Modulation (AM) sound broadcasting. The band used by the MF NAS in Australia is part of
the MF Broadcasting Service allocation in Region 2 as defined by the International
Telecommunication Union (ITU)2. This means MF broadcasting receivers capable of
receiving the frequencies used by the MF NAS are commercially available.
Australia is located in ITU Region 3 where the band has been allocated to the Fixed, Mobile,
Radiolocation and Radionavigation services. Stations in the MF NAS operate in accordance
with the conditions that apply to the primary allocated services in the band3. MF NAS
transmitters must therefore operate at lower power levels than are typically used by MF
sound broadcasting transmitters. MF NAS transmitters are limited to a transmitter power of
400 Watts.
The MF NAS does not have the same audio quality, level of interference protection or a
defined protected coverage area given to MF sound broadcasting stations. The intended
coverage of a MF NAS transmitter is described in the apparatus licensing information for
Narrowband Area Service Stations [1] to be within 10 km of the transmitter. Actual
coverage however depends on background noise levels and the number of adjacent MF NAS
stations.
1
There are existing MF NAS stations providing commercial broadcasting services under licences issued by the
Australian Broadcasting Authority under section 40 of the Broadcasting Services Act 1992. Narrowcasting is
defined in section 18 of the Broadcasting Services Act 1992.
2
ITU Region 2 includes the continents of North and South America and surrounds.
3
See the note under subsection 10 (3) of the Australian Radiofrequency Spectrum Plan.
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Spectrum Planning Report SP 5/04 1 April 04
This report utilises the data and planning methods used in the MF sound broadcasting
service to determine a worst case value for the MF NAS co-channel coordination check.
3. MF BROADCASTING SERVICE PLANNING
MF broadcasting services planned by the Australian Broadcasting Authority (ABA) are
designed to provide program delivery to a defined licence area. The methods used by the
ABA in planning MF broadcasting services are set out in the ABA Technical Planning
Parameters and Methods for Terrestrial Broadcasting4 [2] and are based on the
recommendations of the ITU.
These planning methods are designed to test whether the characteristics of the transmitter
using a particular frequency can be found to fall within the range of permitted maximum
transmitter power and the level of interference and noise. The initial planning task is to
determine the levels of field strength required to overcome the individual sources of
interference and background noise at sites within the licence area.
These individual protected field strength levels are the field strength levels required at the
receiver to overcome the individual levels of interference or noise source in isolation. These
levels are then combined using a root square method which excludes sources that are less
than 50% of the running total. This method of combining the protected field strength levels
is set out in part 5A.10 “Calculation of Usable Field Strength- Summation of Interference”
of the ABA Technical Planning Parameters [2].
MF broadcasting stations are planned for a particular licence area and so can take into
account local conditions of surface conductivity, electromagnetic noise and actual
interference signal levels from other transmitters which arrive at the receiver via both
ground-wave and sky-wave propagation. See Appendix 6 for a discussion of MF
propagation modes. Taking local conditions into account requires detailed planning. The
MF NAS over-the-counter checks in contrast, use worst case values for these factors.
4. DETERMINING AN MF NAS CO-CHANNEL REUSE DISTANCE
To determine the co-channel reuse distance it is first necessary to determine the maximum
unwanted signal protected field strength level at the receiver. To determine this level the
following information is required:
• the minimum wanted signal level;
• the maximum protected level of the unwanted signal;
• the sources of interference;
• any allowance for multiple sources of interference; and
• the required level of co-channel protection.
The maximum interference contribution from each co-channel source can then be calculated
and the required minimum propagation distance between the receiver and the unwanted
signal source determined. This distance is then added to the distance of the receiver from the
wanted signal transmitter to determine the co-channel separation distance between
transmitting stations.
4
Referred to in abbreviation in this report as the ABA Technical Planning Parameters.
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Spectrum Planning Report SP 5/04 2 April 04
4.1 Minimum Wanted Signal Level
The minimum wanted signal level for receivers in the MF NAS has not been defined
although adequate signal levels based on man-made noise levels are referred to in the
Narrowband Area Service Stations licence information [1] in regard to procedures for site
relocation of MF NAS transmitter licences. The minimum wanted signal level at the
receiver is dependent on receiver sensitivity, man-made noise levels and atmospheric noise
levels as used in MF broadcasting service planning and an allowance for the contribution of
interference. A minimum wanted signal for stations in the MF NAS has been derived in
Appendix 1.
Levels for the minimum wanted signal level are calculated in Appendix 1 for urban,
suburban and rural levels of man-made noise. From these levels a minimum wanted signal
level of 76.4 dBuV/m has been selected. This value corresponds to that calculated based on
suburban man-made noise levels. This value leads to a coverage range closest to the 10 km
coverage range expressed in the licensing information for Narrowband Area Service
Stations.
4.2 Calculation of the Protected Unwanted Signal Level
The maximum unwanted signal protected field strength level at the MF NAS receiver is
based on the minimum wanted signal level at the edge of coverage. It is the protected field
strength level of the unwanted signal that will result in the 1 dB interference allowance over
the combined noise protected field strength level used to calculate the minimum wanted
signal level. An unwanted signal protected field strength level 6 dB below the level of the
combined noise protected field strength will lead to a 1 dB increase in the total protected
field strength level requirement at the receiver. See part 5A.10 of the ABA Technical
Planning Parameters [2].
Minimum Wanted Signal - 1 dB Interference Allowance = Protected Combined Noise
Protected Combined Noise - 6 dB = Maximum Unwanted Signal Protected Field Strength
76. 4 dBuV/m - 1 dB - 6 dB = 69.4 dBuV/m
4.3 Sources of the Unwanted Signal
MF sound broadcasting planning uses detailed information regarding each existing station to
determine the level of interference to receivers in the coverage area of the proposed new
transmitter. The level of interference from other MF sound broadcasting stations can vary
across a wide range of values due to differences in the radiated power of potentially
interfering transmitters, and distance of those transmitters from the licence area of the
proposed transmitter.
The interfering signal sources considered in MF sound broadcasting include:
• Co-channel interference from the wanted station sky-wave propagated signal;
• Co-channel interference from unwanted stations ground-wave propagated signal;
• Co-channel interference from unwanted stations sky-wave propagated signal;
• Adjacent channel interference levels; and
• Receiver image frequency interference levels.
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Spectrum Planning Report SP 5/04 3 April 04
Not all of these sources of interfering signal are relevant to the determination of the MF NAS
minimum co-channel separation distance.
For example the localised nature of MF NAS coverage, intended to be typically within
10 km from the transmitter, means that co-channel interference from the wanted stations own
sky-wave propagated signals does not contribute significantly to the level of unwanted
signal. This is because of the height of the E-layer (100 km) and the null in the vertical
antenna radiation pattern directly above the antenna mean that little radiated energy reflects
back into the coverage area. See Appendix 6 for a discussion of MF propagation modes.
4.4 Allowance for Multiple Interference Sources
Unlike MF sound broadcasting planning, where individual levels of interference from each
source need to be calculated, the development of a worst case value for the minimum co-
channel separation distance for MF NAS transmitters can be based on a common set of worst
case parameters for all interference sources. For example MF NAS stations can be assumed
to have the same worst case level of radiated power. Such an analysis will therefore place all
co-channel interfering stations at the same minimum separation distance from the proposed
transmitter.
It can be shown (see Appendix 2) that in the worst case scenario there will be a maximum of
6 adjacent co-channel transmitters at the minimum separation distance. If the maximum
protected unwanted signal level was based on a simple logarithmic summation of the
contribution of these six transmitters then, each transmitter would contribute a level of signal
7.8 dB = 20 * Log10 (6) lower than the maximum protected unwanted signal level
previously calculated.
However using the interference summation method set out in Part 5A.10 of the ABA
Technical Planning Parameters [2] previously described, only the first of a set of equal level
interference sources of the same level will be added to the combined protected noise level. If
the set of equal interference levels are 6 dB below the level of the combined protected noise
level once the first is added to the combined protected signal level then subsequent
interference sources will then be 7 dB below the sum total and hence ignored as they will not
significantly add to the running total.
The different types of unwanted signal can be considered and combined in the same way.
This includes co-channel, adjacent channel and image frequency interference sources. The
protected field strength level assigned to each interference source is based on a common
level of impact on the audio output quality of the receiver. The protected unwanted signal
level calculated in the previous section can therefore be considered as coming from a single
source, of the type of interference that is of interest. In this case co-channel interference.
4.5 The Level of Protection from Co-Channel Sources
The level of protection given to the wanted signal from co-channel interference in the
MF NAS has been taken as 30 dB, the same level specified in MF sound broadcasting
planning. The narrow bandwidth emission characteristic of the MF NAS will not directly
affect the co-channel protection ratio as the receivers are typically MF sound broadcasting
receivers.
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Spectrum Planning Report SP 5/04 4 April 04
Information regarding the affects of program material type on the necessary protection ratio
is given ITU-R BS.560-3 “Radio-Frequency Protection Ratios in LF, MF and HF
broadcasting” [4]. No location or time variability requirement is specified for MF NAS so
median signal levels have been assumed for both the wanted and unwanted signals. The
selection of the 30 dB protection ratio allows the use of existing MF sound broadcasting
planning information.
The maximum unprotected field strength level of the unwanted signal is:
69.4 dBuV/m – 30 dB = 39.4 dBuV/m
4.6 Calculation of the Minimum Co-Channel Separation Distance
The minimum separation distance between the receiver and the unwanted co-channel MF
NAS station may be determined assuming homogeneous surface conductivity over that
distance using the ground-wave propagation curves in ITU-R P.368-7 “Ground-wave
Propagation Curves for Frequencies between 10 kHz and 30 MHz” [3].
These curves have been used in preference to the curves in the ABA Planning Parameters [2]
as the curves in Figure 36 of ITU-R P.368-7 (Appendix 5) are closer in frequency to that
used by the MF NAS. The ABA curves are based on the data and methods used in ITU-R
P.368-7 but have been redrawn for frequencies in the MF sound broadcasting band in
Australia.
The curves in Figure 36 of ITU-R P.368-7 are for 1 kW effective monopole radiated power.
That is the equivalent radiation produced by feeding a short vertical monopole antenna on a
perfect ground with 1 kW. This results in a 300 V Cymomotive force at a distance 1 km
from the antenna.
The maximum transmitter power specified for MF NAS stations is 400 W however the
antenna gain requirement is less specific. Typically the gain of the antenna used is no
greater than that of a short vertical monople reference antenna5. The licensing information
for the Narrowband Areas Service Stations [1] states that licensees may use antennas other
than vertical radiators provided they produce a sky-wave component no greater than for a
vertical monopole radiator.
The worst case value for the radiated power for a MF NAS station has been taken as 400 W
effective monopole radiated power (e.m.r.p.). In order to use the curves in figure 36 of ITU-
R P.368-7 the maximum unwanted signal level must be increased by 10 Log10 (1000/400) =
4 dB. The required minimum co-channel separation distance between a receiver at the edge
of coverage and the unwanted station transmitter is:
Distances from Figure 36 of ITU-R P.368-7
Surface Conductivity Surface Conductivity
Unwanted Adjusted Field Strength 1 mS/m 30 mS/m
39.4 dBuV/m 43.4 dBuV/m 35 km 145 km
5
The wavelength of frequencies used in the MF NAS is around 180 metres. MF NAS transmitting antennas
are typically less than ¼ wavelength in size. There are exceptions however, where MF NAS applicants have
gained shared access to MF broadcasting antenna but even in these cases other factors such as antenna trap
efficiency, additional cable losses and the limit increase in gain at these wavelengths mean that the radiated
power is unlikely to be significantly greater than 400 We.m.r.p.
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Spectrum Planning Report SP 5/04 5 April 04
This distance between the receiver and the unwanted signal transmitter must now be added
to the distance of the receiver from the wanted signal transmitter. This distance is calculated
in Appendix 1 as 17 km. The worst case minimum separation between co-channel MF NAS
transmitters is simply calculated as 145 + 17 = 162 km.
The minimum co-channel separation distance calculated is based on the suburban man-made
noise levels. If urban man-made noise based values are used then the minimum separation
distance falls to 125 km. If rural man-made noise based values are used then the minimum
separation distance rises to 171 km.
5. DISCUSSION
It is apparent from the distances calculated in Section 4.6 and in Appendix 1 that the
variability of MF surface conductivity across Australia leads to a wide range of possible
values for both the MF NAS coverage distance and the minimum co-channel separation
distance. The spread of values for the minimum co-channel separation distances calculated
from the field strength curves of ITU-R P.368-7 ranges from 39 km to 162 km.
This range of distances reflects the fact that variation in ground conductivity can lead to up
to 30 dB variation in field strength at a given distance from the transmitter in different areas.
The lack of detailed MF surface conductivity mapping and the assumptions made in this
analysis means that in practice the full range of flexibility in the minimum co-channel
separation distance would not be available.
Night time sky-wave propagation of unwanted signals from adjacent co-channel transmitters
can potentially lead to higher levels of interference at night, see Appendix 6. In Appendix 6
worst case predicted field strength levels for sky-wave propagated emission has been
calculated for distances from 100 to 160 km and show little variation in level. The worst
case minimum separation distance required for sky-wave signals to achieve the unprotected
co-channel unwanted signal level of 39.4 dBuV/m would be over 500 km.
The current planning for MF NAS obviously does not consider sky-wave interference. This
is a reasonable approach for a non broadcasting service. The calculated levels of sky-wave
propagated signals in Appendix 6 indicate that there is little variation in the level of signal
for distances between 100 km and 160 km. So if applicants were able to request a variation
of the minimum co-channel separation distance based on an engineering assessment then
there would not be a significant increase in the level of sky-wave interference.
Where the worst case minimum co-channel separation distance is not met, and both the
existing transmitter and the proposed transmitter are located in urban areas, then there is also
scope for accepting a reduced minimum co-channel separation. The coverage distance of
transmitters in urban areas having already been reduced by man-made noise levels. In this
case the worst case minimum co-channel separation falls to a distance of a 125 km.
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Spectrum Planning Report SP 5/04 6 April 04
6. CONCLUSION
There is scope to allow for the variation of the 160 km value for the co-channel minimum
separation distance between MF NAS transmitters as the value is based on worst case
propagation and noise conditions. The use of this scope to give potential licensees greater
flexibility will however come at a cost, potentially to both the licensee and the ACA.
That cost would be that of the additional engineering studies and resources necessary to
determine whether a particular value is acceptable in the case of a particular licence
application. To allow applicants to request a variation from the currently used worst case
value would change the nature of the licensing arrangement from an “over-the-counter” to
one more reliant upon case-by-case engineering assessment.
BIBLIOGRAPHY
[1] ACA Website information on Apparatus Licensing of Narrowband Area Service
Stations (April 2004)
[2] ABA Technical Planning Parameters and Methods for Terrestrial Broadcasting
(April 2004)
[3] ITU-R BS.560-3 Radio-Frequency Protection Ratios in LF, MF and HF broadcasting
[4] ITU-R Recommendation 368-7 “Ground-wave Propagation Curves for Frequencies
between 10 kHz and 30 MHz”
[5] ITU-R P.382-2 “World Atlas of Ground Conductivities”
[6] ITU-R P.1147 “Prediction of Sky-wave Field Strength at Frequencies between about
150 and 1700 kHz”
[7] ITU-R P.341-3 “The Concept of Transmission Loss for Radio Links”
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Spectrum Planning Report SP 5/04 7 April 04
Appendix 1
Appendix 1 – MF NAS Wanted Signal Level and Coverage
Background
New MF NAS stations do not have a specified minimum wanted signal level or defined
service area or protected coverage. The licensing information paper indicates that the
typically coverage provided by an MF NAS transmitter lies within 10 km of the
transmitter but that actual coverage can be slightly less or extend considerably further
depending on the level of electromagnetic noise, and the number of surrounding MF NAS
transmitters.
Factors in Determining the Minimum Wanted Signal Level
A minimum wanted signal level can be determined from receiver sensitivity, the level of
electromagnetic background noise and an allowance for interference.
Receiver Sensitivity
The ABA planning guidelines specify that in the absence of man-made noise, atmospheric
noise and interference from other services, a minimum field strength of 54 dBuV/m is
required to meet the required audio quality from an AM sound broadcasting receiver see
Part 5A.15.6 of the ABA Technical Planning Parameters [2].
Electromagnetic Noise
Electromagnetic noise levels affect the coverage of an MF NAS transmitter by raising the
noise floor and hence increasing the required field strength of the wanted signal at the
receiver. This electromagnetic noise includes both man-made noise and atmospheric
noise.
The protected field strength levels required to overcome man-made noise are specified in
the ABA planning parameters for the MF broadcasting service as:
Urban areas 10.0 mV/m which is equivalent to 80 dBuV/m;
Suburban areas 2.5 mV/m 68 dBuV/m;
Rural areas 0.5 mV/m 54 dBuV/m
The level of atmospheric noise at MF frequencies varies across Australia. The method of
calculating the level of field strength required to overcome atmospheric noise levels at MF
frequencies is described in part 5A.8 of the ABA Technical Planning Parameters.
The minimum protected field strength level required in any particular area is given by the
formula:
A = Fam – 6.5 – 20 * Log10 (frequency MHz)
Where the value of Fam is determined from Table 5A.3 of the ABA Technical Planning
Parameters and reproduced in Appendix 3.
Over continental Australia and Tasmania the value of Fam varies between 52 and 85.5 dB.
Fam A (Minimum protected field strength)
Southern Tasmania 52 41
60 49
70 59
80 69
Northern Australia 85.5 dB 74.5 dBuV/m
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Spectrum Planning Report SP 5/04 8 April 04
Appendix 1
Allowance for Interference
Typically in the calculation of minimum wanted signal level an allowance is made for the
effect of interference. An allowance of 1 dB for interference has been used in this paper.
This value is a reflection of the outcomes of the method for combining signal levels set
out in Part 5A.10 “Calculation of Usable Field Strength-Summation of Interference” of the
ABA Technical Planning Parameters [2]. This part specifies that the calculation of the
usable field strength in the presence of a number of sources should be performed by
considering the protected field strength resulting from each individual source of
interference acting alone.
The individual field strength levels should be taken in order of descending magnitude
adding the squares of the field strengths and extracting the square root of the sum
excluding the components which are less than 50% of the total. The summation of two
variables with a 6 dB difference in level leads to a 1 dB increase in the total value above
that of the larger of the two values.
Minimum Wanted Signal Level
For the worst case analysis being used in this paper for determining the minimum
separation distance between MF-NAS transmitters the protected minimum field strength
level related to receiver sensitivity, man-made and atmospheric noise have been combined
using the method in the ABA Technical Planning Parameters and a 1 dB allowance for
interference added as described above.
Receiver Sensitivity Man-made Noise Atmospheric Noise Combined
Urban 54 dBuV/m 80 dBuV/m 74.5 dBuV/m 82.0 dBuV/m
Suburban 54 68 74.5 76.4
Rural 54 54 74.5 75.5
Coverage
The MF NAS relies on ground-wave propagation of radio waves for coverage. Sky-wave
propagation at MF frequencies is only effective at night. The combined field strength
levels calculated above can be used to determine the coverage range provided by an
MF NAS transmitter in the absence of interference from other MF NAS stations using the
ground-wave propagation curves in Figure 36 of ITU-R Recommendation P.368-7
reproduced in Appendix 5.
The surface conductivity of the Earth varies significantly at MF frequencies and this
variation is not directly related to measured soil conductivity. ITU-R Recommendation
P.382-2 “World Atlas of Ground Conductivities” contains information regarding the level
of variation of surface conductivity at MF frequencies across the Australian continent.
Typical values for surface conductivity over continental Australia and Tasmania range
between 1uS/m – 30 uS/m. See figure 39 of ITU-R P.382-2 reproduced in Appendix 4 for
details. This can lead to a variation of up to 30 dB in the field strength between points at
the same distance from the transmitter in different areas. See figure 36 of ITU-R P.368-7
reproduced in Appendix 5.
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Spectrum Planning Report SP 5/04 9 April 04
Appendix 1
The curves in ITU-R P.368-7 are for an effective monopole radiated power of 1 kW. That
is the equivalent radiation produced by feeding a short vertical antenna with 1 kW. This
results in a cymomotive force of 300 V. The technical requirements for the MF NAS
specify a transmitter power limit of 400 W.
While the use of an antenna of a specified gain by MF NAS transmitters is not required
applicants are encouraged to utilise the best antenna available. Typically the gain of the
antenna used is no greater than that of a short vertical monople reference antenna6. The
variation in gain of MF antenna with wavelengths less than ¼ wavelength or 90 electrical
degrees is relatively small, typically less than 0.5 dB. The worst case value for the
radiated power for a MF NAS station has been taken as 400 W effective monopole
radiated power (e.m.r.p.).
The calculated field strength of the wanted signal must be increased by
10 * Log10 (1000/400) = 4 dB in order to make use the curves in figure 36 of
ITU-R P.368-7.
Distances from Figure 36 of ITU-R P.368-7
Surface Conductivity Surface Conductivity
Combined Adjusted Field Strength 1 mS/m 30 mS/m
Urban 86 dBuV/m 3.0 km 11 km
Suburban 80.4 dBuV/m 4.2 km 17 km
Rural 79.5 dBuV/m 4.8 km 20 km
6
The wavelength of frequencies used in the MF NAS is around 180 metres. While MF NAS applicants are
encouraged to use the best antenna available to them most MF NAS antenna are less than ¼ wavelength in
size. There are exceptions, where MF NAS applicants have gained shared access to MF broadcasting
antenna but even in these cases the radiated power is unlikely to be substantially greater than 400 W.
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Spectrum Planning Report SP 5/04 10 April 04
Appendix 2
Appendix 2 – Consideration of Interference from Multiple Co-
Channel Transmitters
Consider the worst case physical layout of stations that is the tightest packing of stations.
On a smooth geometric plane with all stations spaced equally at the minimum separation
distance in a grid see Figure 1.
D01-sc
There can be no more than six other stations at this distance at the minimum distance from
any station. This derives from the geometric relationship between the radius of a circle
and its circumference. C = 2 * Pi * R or 6.28 * R
The next closest stations are twice as far away and hence contribute 6 dB less signal field
strength. If we ignore the more distant stations then the maximum level of unwanted field
strength will be 10 * Log10 (6) = 7.8 dB greater than the contribution of any one station.
However the ABA method set out in Part 5A.10 “Calculation of Usable Field Strength-
Summation of Interference” of the Technical Planning Parameters includes a 6 dB
exclusion principle. Utilising this principle means that the contribution from multiple
equal level interference sources cannot exceed the interference contribution of any one
station by more than 6 dB to be significant.
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Spectrum Planning Report SP 5/04 11 April 04
Appendix 3
Appendix 3 – Extract from the ABA Planning Parameters
Table 5A.3
Values of Atmospheric Noise Levels
Fam (dB): 1600-2000 hours, LT, Summer
Lat Longitude (oE)
o 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
S
0 72 72 73 73 73 70 69 69 69 67 67 65 63.5 62 60 59
5 72 74 75.5 77 77 76 75 75 75 75 75 72 70 69 67 60
10 71 75 78 80 82 84.5 85.5 85 83.5 82 80.5 78 75 73 72 71
15 67 70 75 80 81 85 85 85 84 82 80 79 78 77 76 76
20 64 67 70 74 75 77 78 78 78 77 77 76.5 76 76 75.5 75
25 61 63 66 68 70 72 73 73 73 73 73 73 73 73 72 72
30 56 60 62.5 64 66 68 69 70 70 70 70.5 71 71 71.5 71.5 71
35 49 52 56 58 60 61 62 64 65 66 67 67.5 68 68 69 67
40 42 45 48 50 43 53 56.5 57 58.5 60 61 62.5 63 63 63 62.5
45 35 37 39 40 43 45 47 50 52 54 55 56 57 59 59 58
50 24 25 27 29 30 35 36 38 40 42 45 46 47 48 49 49
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Spectrum Planning Report SP 5/04 12 April 04
Appendix 4
Appendix 4 – Extract from ITU-R P.382-2
FIGURE 39
Australia
115° 120° 125° 130° 135° 140° 145° 150°
10
3
15° 15°
1 1
10
10
20° 20°
30
3 30
30 3 1
10 30 10
10
25° 25°
10 3
10
3
30° 30
30°
10 10
10
3
1 30
30
10 30 10
35° 35°
3
1
10
3 30 3
40° 40°
3 30
10
115° 120° 125° 130° 135° 140° 145° 150°
0832-39
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Spectrum Planning Report SP 5/04 13 April 04
Appendix 5
Appendix 5 – Extract from ITU-R P.368-7
D36-sc
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Spectrum Planning Report SP 5/04 14 April 04
Appendix 6
Appendix 6 – MF Propagation
Ground-wave Propagation
Radio energy leaving the transmitting antenna as ground-waves remains in intimate
contact with the surface of the Earth. Ground-waves at MF frequencies are not limited to
line-of-sight but are able to traverse over and around large terrain obstacles with relatively
little additional loss. MF ground-waves do however attenuate quickly with increasing
distance from the transmitter particularly over dry land.
ITU-R Recommendation 368-7 “Ground-wave Propagation Curves for Frequencies
between 10 kHz and 30 MHz” provides the basis for the calculation of the field strength of
ground-wave emissions at distances greater than 1 km from the antenna. The intimate
contact of the radio wave with the surface of the earth leads to surface conductivity being
a dominant factor in the determination of the field strength of a ground-wave at any
particular distance from the transmitter antenna.
The surface conductivity of the Earth varies significantly at MF frequencies and this
variation is not directly related to measured soil conductivity. ITU-R Recommendation
P.382-2 “World Atlas of Ground Conductivities” contains information regarding the level
of variation of surface conductivity at MF frequencies across the Australian continent.
Transmitters in the MF-NAS rely on ground-wave propagation of signals to receivers.
Typical values for surface conductivity over continental Australia and Tasmania range
between 1uS/m – 30 uS/m (See figure 39 of ITU-R P.382-2 reproduced in Appendix 4).
This can lead to a variation of up to 30 dB in the field strength between points at the same
distance from the transmitter in different areas (See figure 36 of ITU-R P.368-7
reproduced in Appendix 5).
Sky-wave Propagation
Radio energy leaving the antenna as a sky-wave travels upward and outward at a range of
angles from the antenna dependant on the antenna vertical radiation pattern. Sky-waves
travel principally through free space and are independent of the ground over which they
travel. When a sky-wave at MF frequencies reaches the ionosphere (a part of the Earth's
atmosphere that typically lies between 80 and 350 kilometres above the Earth's surface) it
can be reflected back towards the Earth's surface. The ionosphere is made up of a number
of different layers. These layers are disrupted by the radiation from the sun and vary in
height and density over the course of a day.
The E layer at a height of around 100 km, plays a significant role in the propagation of
radio waves at MF frequencies particularly after sun set. Sky-waves are important
because they lose their energy more slowly than ground waves and, as they are reflected
from the ionosphere high above the rough surface of the earth, they are capable of
travelling long distances.
Although E layer reflection of sky-wave signals is important at MF frequencies it is
generally only considered in respect to its interference potential to the ground-wave
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Spectrum Planning Report SP 5/04 15 April 04
Appendix 6
delivered service. This is because of variation in the level of signal reflected from the
layer across the day as the E layer dissipates during daylight hours.
ITU-R Recommendation P.1147 “Prediction of Sky-wave Field Strength at Frequencies
between about 150 and 1700 kHz” [6] sets out the modelling and equations required to
determine the field strength of sky-wave radio signals at distances beyond 1 km from a
transmitter. The modelling takes into account attenuation due to absorption by the
ionosphere and local surface conductivity at the transmitter in determining the field
strengths at various great circle distances from the transmitter.
The level of sky-wave signal depends on a number of variables including geomagnetic
latitude, time of the day, sun spot number, sea gain in addition to distance and frequency.
The following formula is from ITU-R P.1147 a number of variables have been set to 0 dB
or replaced by constants in line with ABA planning methods.
Annual median night-time field strength
The predicted sky-wave field strength is given by:
E = V + GS – Lp + A – 20 Log10 P – La – Lt – Lr (1)
where:
E : annual median of half-hourly median field strengths (dB(µV/m)) for a given transmitter
cymomotive force, V, and at a given time, t, relative to sunset or sunrise as appropriate
V : transmitter cymomotive force (dB above a reference cymomotive force of 300 V)
GS : sea-gain correction (dB) (Can be set to 0 dB)
Lp : excess polarization-coupling loss (dB) (Can be set to 0 dB)
A : a constant. At MF, A = 107 except for propagation paths whose midpoints are situated in the part
of Region 3 south of parallel 11° S. In those cases, A = 110
P: slant path distance. P=(d2 + 4h2)1/2 where d = the distance between the transmitters and h = 100
km
La : loss factor incorporating effects of ionospheric absorption and related factors ( = k (p/1000)1/2
above 1600 where k = (2π + 4.95 tan2 (geomagnetic latitude))
Lt : hourly loss factor (dB) (3 dB for 2 hrs after sunset)
Lr : loss factor incorporating effect of solar activity (0 dB).
E = V + 0 – 0 + 110 – 20 log p – 10.29( p / 1000)1/2 – 3 – 0
E (dBuV/m) = V + 107 - 20 log p – 10.29( p / 1000)1/2
For V = - 4 dB (A worst case assumption)
Propagation Distance Field Strength
D = 160 km E = 49.6 dBuV/m
D = 150 km E = 49.9
D = 140 km E = 50.2
D = 120 km E = 50.7
D = 100 km E = 51.1
The co-channel sky-wave interference protected field strength at 145 km is 80 dBuV/m.
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Spectrum Planning Report SP 5/04 16 April 04